Neutron Radiography of Electrochemical Systems - Detailed information

Neutron Radiography group is involved in various projects focused on investigating fuel cell and electrolyser operation investigation. Another area of investigation includes material development such as synthesis of porous materials.

Contents

Neutron Imaging of PEFC Fuel Cells

Working Principle

Polymer electrolyte fuel cells (PEFCs) produce electrical energy using hydrogen as a fuel. Due to electrochemical reaction Hydrogen cations bond with Oxygen anions forming water as a emission product. Fuel cells have already been used in automotive industry as units installed in buses and personal cars. Even though efficiency of fuel cells is relatively high, high stack costs and lack of refueling stations is still an obstacle which makes lots of room for research.

Distinction of super-cooled water and ice in Fuel Cell

It is possible to image areas of super-cooled water and ice in fuel cell using Time of Flight (ToF) neutron imaging technique.

Rotating chopper disk in the beam provides short pulses of neutrons.

Aluminium container with sections containing different water thickensses was used for neutron imaging experiments. 4 sections contain water thicknesses of respectively 125, 250, 500, 1000 um. Ice was produced at -13°C. Note volume expansion in the section b) compared to a).

Upon analysis of the obtained images we conclude that this way of imaging water/ice in the fuel cells is reliable for thicknesses 250um and above.

Materials Development

Development of Gas Difusion Layers for Fuel Cells

Polymer electrode fuel cells (PEFCs) generate electricity and water by reacting H2 and O2. The polymer electrolyte membrane (PEM) acts as an impermeable barrier for the gasses allowing two half cell reactions to occur. Hydrogen oxidation (anode) and oxygen reduction (cathode) take place at the catalyst layers (CLs), while the electrons produced travel throughout an external circuit. In order to reach the CLs, H2 and O2 diffuse through porous carbon materials, so-called gas diffusion layers (GDLs).

To avoid the problem of the water flooding we propose to use a modify GDL with preferential water pathways, created by modifying the hydrophilicity of selected areas of porous substrate by radiation grafting.

Syntheis through radiation grafting

Grafting consist on a copolymerization where the second monomer polymerize attached to the surface of a primer polymer. In our case we use an electron beam radiation to generate the radical anchor site where the copolymerization takes places. Blocking the radiation source with masks leave only the expose areas activated therefore susceptible of modification, allowing the mask design to determine the pattern on the porous material.

GDL are generally highly porous carbon papers coated by a fluoropolymer to enhance hydrophobicity. Modifying the hydrophobicity of this coating can create the separate gas and water channels

The possibility to create a pattern depends on the coating distribution and the porous material microstructure. There is a need to have a enough coating coverage of the GDL fibers, to ensure continuity in the modification, as it can be seen in the cross section SEM picture (a) of a Toray carbon paper in-house coated with FEP. The hydrophilic pattern footprint can be characterize by EDX elemental mapping (b).

Wetability tuning

The adequate synthesis conditions were obtain using simplified systems. Flat films of fluorinated ethylene propylene (FEP) were modify as in the porous material. Two different grafting polymers were employed: Acrylic acid (AA), in blue, is a more economical option but it gives high grafting degree (DG) and there are indications that the modification also affects the bulk of the material (a), yielding a contact angle of 50°(b) and the grafting N-vinylformamide (NVF), in red, reaches a plateau after 30 minutes (c), indicating a surface modification that yields a lower contact angle of 20

Conclusions

It is possible to modify the hydrophobicity of the materials using rather short reaction times. The radiation grafting method is compatible with local modifications

The studies on flat surfaces are transferable to porous media, allowing an easier and faster optimization process

At high current density, fuel cell performance is improved by using GDL with patterned wettability

Neutron Imaging of PEM Water Electrolyzer

Working Principle

PEM (Proton Exchange Membrane) Water Electrolyzers are used to produce hydrogen out of water. Water is split into Oxygen, electrons and protons when certain voltage is applied. Protons are then transported through Proton Exchange Membrane to the cathode side where they form Hydrogen. Producing Hydrogen can be considered as a way of storing energy which is one of the biggest issues to tackle in nowadays power engineering. Combining electrolysis with renewable sources of energy may also serve as a way of stabilizing the grid and protecting it from losses caused by overproduction due to wrong energy demand predictions.

Our test bench

Mobile electrolyzer test station which can be easily transported to the beamline in order to perform neutron imaging experiments. Bench is built in a way which enables user to easily modify setup. Current electrolyser cells are designed for membranes with active areas of 1cm x 1cm. Test bench can handle up to 20A current demand and is high frequency resistance measurements ready.

Electrolyzer research

Current study focuses on water distribution in porous media of Electrolyser. Influence of extensive oxygen evolution is still unclear, it can be both good or bad. One one side produced gas flowing to the exhaust might enhance flow of the water which could positively affect reaction but on the other hand presence of oxygen limits access of water to catalyst layer.

Already performed experiments included neutron imaging of operating electrolyser under various current densities. Image bellow shows water/gas distribution for current densities from 0A to 2.5A.

It appears that current density has no influence on the gas distribution on the anode side (where water is oxidized).